Laser distance measurement devices, which measure the distance to a target by using a laser beam, are used for detecting obstructions for the purposes of for example driver assistance, parking assistance, etc. for automobiles etc. Also, laser distance measurement devices are used for surrounding monitoring such as object detection etc. existing between vehicles and doors at platforms of railroads. Further, laser distance measurement devices are sometimes used for scoring in competitions such as artistic gymnastics etc.
As a method of measuring the distance to a target by using a laser distance measurement device, there is a method in which two laser distance measurement devices are arranged face to face with the target between them so as to measure the distances to the target. According to this measurement method, while a first laser distance measurement device is measuring the distance to the target, the second laser distance measurement device can simultaneously measure the distance on the back side of the target, which the first laser distance measurement device is not allowed to measure.
However, when two laser distance measurement devices are arranged face to face so as to measure the distances to a target, a laser beam emitted from one of the laser distance measurement devices is sometimes received by the other laser distance measurement device. In such a case, the other laser distance measurement device falsely calculates the distance to a target that does not actually exist. As a technique for suppressing such false detection of targets, a method is known in which the rotation speed of a rotation mirror is adjusted on the basis of the rotation angle of the rotation mirror that controls the emission timing of a laser beam in each of the two laser distance measurement devices (see Document 1 for example). In this technique, one of the laser distance measurement devices determines the presence or absence of a possibility of false detection of a target on the basis of information representing a relationship between the rotation angle of the rotation mirror of the device itself, the rotation angle of the rotation mirror of the other laser distance measurement device, and the rotation angle at which the target is falsely detected. Also, when there is a possibility that a target will be detected falsely, the laser distance measurement device adjusts the rotation speed of the rotation mirror so as to reduce the possibility of false detection of the target.    Document 1: Japanese Laid-open Patent Publication No. 2011-112503